Upload
others
View
3
Download
0
Embed Size (px)
Citation preview
Thin Film Gas SensorsThin Film Gas Sensors
Mohammad F. AlMohammad F. Al--KuhailiKuhailiPhysics Department Physics Department –– KFUPM KFUPM
October 2003October 2003
Part I : Sensors
Part II : Thin Film Gas Sensors
Part III : TFGS Research at CAPS
Part I
Sensors terminology & specifications
Sensor materials
SENSORS
TERMINOLOGY
&
SPECIFICATIONS
SENSOR TERMINOLOGY
Sensor: a device that detects or measures a physical quantity. Usually the output of a sensor is an electrical signal.
Actuator: a device which converts a signal (usually electrical) to some action (usually mechanical)
Transducer: a device that converts energy from one form into another
Sensors and actuators are forms of transducers.
sensor actuator
operator input
power source
signalprocessor
input output
Inpu
t for
m o
f ene
rgy
signal signal
Types of Sensors
Active Sensor: one that can generate a signal without the need for any external power supply (photovoltaic cell).
Passive Sensor: needs an external source of energy (electrical).
The forms of energy involved in the sensing operation can be conveniently divided into six categories:
Chemical (concentration, reaction rate)
Electrical (current, voltage, resistance)
Magnetic (moment, permeability)
Mechanical (position, force, stress)
Radiant (energy, phase, polarization)
Thermal (heat, temperature)
Sensor Requirements
Low cost
Structurally & chemically reliable
Selectivity
Sensitivity
Ruggedness
Sensed QuantitiesStrain & pressure
Position, direction, distance & motion
Light & associated radiation
Temperature
Sound, infrasound & ultrasound
Liquids & gases
Environment (moisture, acidity, radioactivity, pollution)
SENSOR
MATERIALS
MetalsMetals consist of fixed ion cores surrounded by a sea of free electrons (free electron gas).
They have closed-packed structures.
They have the following properties:
• High electrical conductivity
• High thermal conductivity
• High strength
Applications
Thermal expansion devices
Shape memory alloys
Thermocouples
Strain gauges
Catalysts
Electrodes (interface to sensing elements)
Semiconductors
Semiconductors are characterized by :
A bandgap in their band structure
Have two types of charge carriers
Can be doped
Can form rectifying or ohmic contacts
This leads to :
Ability to control their conductivity
Ability to modify their structures
Ability of semiconductors to respond to light
Applications
Thermistors
Optical detection
Gas-sensitive resistors
Charge-coupled devices
Position and displacement sensors
DielectricsHave high electrical resistivity
Can be polarized in an electric field
Dielectrics include the following classes:
• Piezoelectric materials: strain – electric field
• Pyroelectric materials: temperature gradient –electric field
• Ferroelectric materials
Examples: Quartz – PZT – Polymers
Applications
• Crystal oscillators• Radiation detection• Vibration monitors• Air bags• Intrusion alarms• ABS in cars• Fuel level monitor• Leak detection• Optical shutters
• NDT• Particle size dist.• Large area display• Ultrasonic imaging• Airport control• Earthquake detection• Touch panels• Thermography• Motion detection
Magnetic MaterialsClasses of magnetic materials:
• Diamagnetic materials
• Paramagnetic materials
• Ferromagnetic materials
• Antiferromagnetic materials
• Ferrimagnetic materials
• Superconductors
These materials respond differently to an external magnetic field.
Applications
Position & movement sensors
Coded sensors (credit cards, smart cards)
Hall effect sensors
SQUIDS: measurement of extremely small magnetic fields (human brain)
Other Materials
Radiant materials (IR detection)
Solid electrolytes
Optical fiber sensor materials
PART II
TFGS
Sensing Mechanism
Sensor Variables
Sensor Parameters
Advantages
Applications
Problems & Limitations
THIN
FILM
GAS
SENSORS
Thin Film Gas Sensors
Thin films of a semiconductor metal oxide (e.g. SnO2) show a substantial conductivity change when only small concentrations of combustible gases (e.g. CO) are present in a large excess of oxygen (e.g. air).
The conductance is controlled by surface processes.
There is a large number of semi-conducting metal oxides sensitive to gas composition in this way.
These include SnO2 , ZnO , WO3 , Fe2O3 , TiO2 , In2O3 and Ga2O3.
These films are sensitive to a range of combustible gases (H2 , CO , and CH4).
SENSING
MECHANISM
Metal – Semiconductor Junctions
Consider the contact between an n-type semiconductor and a metal where χ < Φm
When equilibrium is reached, the band diagram looks like
There will be a barrier to electron flow from semiconductor to metal of magnitude:
ΦSB = Φm – χ { Schottky barrier }
Additionally, there will be a built-in potential of
magnitude:
eVo = ΦSB – EFSC
Forward bias increases S – M current (reduce eVo)
Reverse bias decreases S – M current (increase eVo)
M – S current is the same (reverse saturation current)
Such a junction behaves as a diode (rectifying)
Behavior of Gas Sensors
Many sensors are made from polycrystalline materials made up of grains.
Unmodified SnO2 is oxygen-deficient and, consequently, is an n-type semiconductor.
The I – V characteristics of SnO2 sensors have shown that the conductivity is determined by a Schottky barrier mechanism.
?This is figure 4.10 in Sensor Materials.However, the correct figure is by McAleer, see page 124 of Solid State Gas Sensors for reference.
Oxygen adsorbed on the oxide surface (grain boundaries) can remove an electron from the semiconductor to from either (O2)-
or O-, so reducing the # of current carriers (oxygen is a surface trap for electrons).
Since the electrons are drawn from ionized donors via the CB, the charge carrier density at the interface is reduced.Therefore, an energy barrier to charge transport ∆E (eVo) is developed
See figure 4 of N. Barsan: J.Phys C Review
As the surface charge grows, the adsorption of further oxygen is inhibited.
At the junction between the grains of the solid, the depletion layer and associated potential barrier make for high resistance.
This can be considered as a back-to back Scottky diode.
Reducing gases (e.g. CO) may then react with the oxygen ions according to reactions such as:(O2)– + 2 CO = 2 CO2 + e–
As a result, the charge carrier density in the n-type semiconductor is replenished (increasing the conductivity) and the height of the Schottky barrier (eVo) reduced.
SENSOR
VARIABLES
SENSOR VARIABLES
1. Sensor material
2. Temperature
3. Analyte gases (composition & concentration)
The major factors affecting the operation of a semiconductor gas sensor are:
Sensor Material
Choice of the material: physical & chemical properties of the sensor
Method of preparation
Crystallinity (single crystal / polycrystalline/ orientation / porosity)
Additives (dopants / catalysts)
Temperature
The gases are discriminated by causing a maximum response at different temperatures.
Maximum sensitivity occurs at different temperatures for different gases and different oxides.
SENSOR
PARAMETERS
SENSOR PARAMETERS
1. Conductance2. Base line3. Selectivity & Sensitivity4. Calibration curve5. Response time6. Sensor geometry7. Reproducibility8. Spread of variables
1. ConductanceConductance = reciprocal of resistance
All the operating characteristics of the sensor are derived from this one measurement
2. Base lineConductance in clean dry air, or humid air of controlled humidity
3. Sensitivity and selectivityS = {[σ (gas) – σ (air)]/ σ (air)} x 100
For a given sensor , Smax and the temperature at which Smax occurs are gas-dependent.
This can be used as the basis for selectivity of a certain gas among a mixture of gases.
Selectivity = sensitivity of gas 1/ sensitivity of gas 2 , for equivalent concentrations of both gases.
4. Calibration curve
- Sensitivity plotted as a function of the concentration.
- The temperature may be set at the value of Smax.
5. Response Time
The time required to achieve (90 % , 70 % or
50 %) of the final change in conductance
following a step change in gas concentration.
Response times are very temperature-dependent (shorter for higher T).
6. Sensor Geometry
The sensor geometry affects the response of the sensor.
The choice of electrodes affects the S-M barrier height.
The placement of the electrodes affects the characteristic reaction depth.
7. Reproducibility
The ability of the sensor to reproduce readings when the same environment is applied to it consecutively under the same conditions.
Sensors are dependent on surface chemical effects which are notoriously irreproducible.
Therefore, the time history of the sensor can severely limit its function.
This effect could be attributed to the adsorption of species on the surface which are not easily desorbed.
Fortunately, for this case, the differences at T > 400 oC are small, and the optimum T (Smax) is 400 – 450 oC.
8. Spread of VariablesThe range of variables taken by a certain parameter (e.g. conductance) for different sensors
Effects of CatalystsCatalysts alter the rate at which chemical
reactions reach equilibrium without themselves
undergoing any permanent chemical change.
Precious metals (Pt & Pd) can modify the
response of TFGS (catalysts).
The effect of the metal catalyst is different for
different gases.
Example: (SnO2 + Pt) CO gas sensor
•SnO2 gas sensor•Pulses of CO gas (15 min)•Dotted line = dry air•Ti = 600 oC (dashed line)•Horizontal = time•Vertical = resistivity
•Same as above but with a Pt catalyst•Resistivity decreases below 150 oC•Size of the response increases as T decreases•The response becomes positive (resistivity increase for higher temperature
The mechanism here is similar to the formation of Schottky barriers from the oxygen surface states except that the charge withdrawal is now due to the high Φm.
Electrons are transferred from CO to Pt, thus reducing the Schottky barrier height and resistivity.
Therefore, SnO2 is less prone to changes in the moisture content of the atmosphere.
Effect of Humidity
Semiconductor gas sensors are sensitive to water vapor and their response to combustible gases is affected by the ambient humidity.Chemisorption of water introduces surface electron states.The effect of water vapor is the increase of surface conductance and the effect is reversible.The effects of water vapor could most certainly account for some of the aging and irreproducibility in practical devices.
ADVANTAGES
ADVANTAGES
Large selection of materials and variables
Possibility of material engineering
Measurable response
Ruggedness – harsh environments
Potential for miniaturization
APPLICATIONS
APPLICATIONS
Automobile engine management – O2
Industrial boiler & furnace control – O2
Refrigerated food storage – NH3
Oil rigs – hydrocarbons & H2S
Domestic gas alarms – hydrocarbons
Steel industry – CO & O2
Chemical industry - hydrocarbons
PROBLEMS
&
LIMITATIONS
PROBLEMS & LIMITATIONS1. Poor understanding of surface & interface chemistry
2. Poor selectivity
3. Temperature instability: turbulent flow
4. Very high (PPB) sensitivity for certain gases
5. Adsorption and desorption of chemical species (long-term effects)
6. Irreproducibility
7. Environmental degradation
8. Water vapor
Typical Gas Sensors
See figure 2 of N. Barsan: J.Phys C Review
PART III
Research on TFGS at CAPS
Some recent results
RESEARCH
on
TFGSat
CAPS
Recently established research on TFGS
Members:• S. M. A. Durrani• E. E. Khawaja• M. F. Al-Kuhaili• E. Bakhtiari
Task: Detection of CO using SnO2
SABIC Project
THE EXPERIMENTAL SYSTEM
Materials : SnO2 , Ga2O3 , HfO2
Different growth conditions:
• Oxygen partial pressure
• Substrate temperature
• Annealing: temperature & atmosphere
Different electrode materials & geometries
Base line measurement
Different experimental parameters
Different gases
Characterization techniques:• Optical• XRD• XPS
REFERENCES
• P T Moseley and A J Crocker, Sensor Materials, IOPP 1996.
• P T Moseley and B C Tofield editors, Solid State Gas Sensors, Adams Hilger 1987.
• I Sinclair, Sensors and Transducers, Newnes 2001.
• D Kohl, J. Phys. D 34 (2001) R125-R149.• N Barsan and U Weimar, J. Phys.: Condens.
Matter 15 (2003) R813-R839.